Method of finishing glass



Aug. 8, 1967 Filed March 16, 1964 gvees Te m pemture In De w. E. SAVAGE in 3,334,452

METHOD OF FINISHING GLASS 2 Sheets-Sheet 2 Plaster Containing O.i2E 37NdHS0 And 2.5% 141,50,

( Based Onwelg'ht Of Plasteri i 2 a 4 5 6 1 a 9 1o 11 Time1n Minutes INVENTOR.

ATTORNEYS United States Patent C) 3,334,452 METHOD OF FINISHING GLASS William E. Savage III, Toledo, Ohio, assignor to Libbey- OWens-Ford Glass Company, Toledo, Ohio, a corporation of Ohio Filed Mar. 16, 1964, Ser. No. 352,165 19 Claims. (Cl. 51-283) The present invention relates broadly to the supporting of articles during processing thereof and more particularly to improved bedding materials for this purpose.

Although in no way restricted thereto, the invention has proved particularly advantageous in securing plate glass blanks that are to be surface finished onto so-called grinding and polishing tables.

By way of illustration, in one well known and widely used type of continuous plate glass production line a series of large metal tables, arranged in end to end relationship, are moved along an endless horizontal path, first beneath a series of heavy iron grinding runners that are fed with sand of progressively decreasing coarseness in a water suspension and then beneath a series of felt polishing runners to which a suspension of rouge is fed.

The rough plate glass blanks to be ground and polished are laid end to end on the upper surfaces of these moving tables so as to present their top surfaces to the abrasive action of the grinding and polishing runners and, to insure the blanks lying level on the table tops and being fixed securely thereto, it is customary to lay them in a bed of plaster.

To this end plaster is usually applied to the tops of the tables in a water slurry and the glass blanks then laid on and rolled into the wet slurry so that when the plaster sets up the glass blanks will be bedded in it and firmly held in the desired position on the tables. Conventional gypsum plasters, or so-called lime or cement-based plasters, have been generally employed for this purpose but none of these commercially available plasters or plaster compositions, whether of the fast or slow setting variety, have ever possessed over-all setting characteristics that are really suitable.

This is because, in laying plate glass blanks, it is important that the plaster in the slurry not begin to set up until after it has been applied to the table and the blank has been laid on and rolled into its desired level position. At the same time it is equally important that the plaster set up rapidly once the glass blank is in place so that the blank will be tightly and immovably secured to the table before it is subjected to the vigorous mechanical abrasive action of the first grinding runner.

It is accordingly a primary object of this invention to provide an improved plaster for use in this connection and elsewhere that will exhibit a relatively slow initial setting action followed by a fast or accelerated final setting action.

Another object is to provide an improved plaster of the above character that will have a bonding action, to surfaces with which it is in contact, that is at least as good as the bonding action of previously known plasters.

A further object is to provide a plaster of this character that has a lower expansion characteristic on setting than the previously known plasters.

Briefly stated, these and other objects, which will become apparent during the course of the following description, are attained by the addition of a special additive or additives to conventionally used and commercially available plasters or plaster compositions.

Specifically, the essential additive of the invention is a soluble sulfite; and, when this is added, with or without an accelerator and with or without a water adsorbent, to conventional plaster combinations it will effectively maintain, or retard where desired, the initial setting period of ice the plaster in a water slurry while, at the same time, accelerating the final set to an extent sufficient to materially shorten the total setting period.

In the drawings, wherein like numerals are employed to designate like parts throughout the same;

FIG. 1 is a plan View of the laying yard and one end of the grinding area of a conventional continuous grinding and polishing line illustrating one use of the special plaster compositions of the invention;

FIGS. 2, 3 and 4 are graphs plotting data obtained by standard tests and showing the extent of retardation of the initial setting of a typical commercial plaster composition obtained with varying amounts of three specific examples of the essential additive of the invention;

FIG. 5 is a graph plotting the temperature rise data for a typical plaster composition, with and without additives of the invention, to show their effect on the over-all setting characteristics.

Referring now more particularly to the drawings, there is illustrated in FIG. 1 a conventional method of bedding rough glass blanks in plaster on grinding and polishing tables just prior to moving the tables beneath the grinding runners of the line.

As there shown, a plurality of tables 10, provided with flat horizontal top surfaces 11, are secured together in end to end relation for movement as a unit along a continuous horizontal path. To this end the tables are customarily bolted or coupled together and mounted on grooves or tracks along which they are moved toward and through the grinding and polishing areas by a pusher mechanism (not shown). A water slurry of plaster is applied to the tops of the tables as they move into the laying yard, designated generally by the letter A, for example by spraying the slurry from nozzles 12.

Each glass blank 13, to be ground and polished, is laid onto the wet plaster in close proximity to a previously laid sheet, by a vacuum frame 14, and .is then rolled into the plaster by a so-called roll-out frame such as illustrated at 15 and which moves independently of the table movement to roll the glass into the plaster bed.

Shortly after passing under the roll-out frame 15 the bedded sheets move beneath the first of a series of rapidly rotating grinding runners 16, the working surfaces of which are fed with a mixture of sand and water through a supply pipe 17 and annular trough 18.

As previously explained it is important that the plaster sprayed on the table tops not begin to set until after the glass blanks have been laid thereon and rolled into the wet plaster but it is equally important that, by the time the glass sheet reaches the first grinding runner 16, the plaster be set up to a point where the glass is tightly adhered to the table and held firmly against the rotating abrasive action of the runner.

This is accomplished according to the present invention by the provision of a special plaster composition that will have a relatively slow initial setting action and an accelerated final setting action or, differently expressed, a relatively long initial setting but a relatively short total setting period.

This special plaster may be either dry formulated in its entirety or formulated by adding the soluble sulfite. of the invention, with or without an accelerator and/ or water adsorbent, to a conventional, commercially available plaster composition.

Most of the early work was with the latter method and the commercially available plaster compositions are gen.- erally found to contain approximately 97% unformulated gypsum plaster and approximately 2.5% portland cement or lime, with other chemicals in amounts from a trace to around 1%.

Regardless of how the special plaster composition is formulated, flexibility in controlling its over-all setting characteristics is greatest when an accelerator is used and this flexibility is achieved by varying the relative amounts of the soluble sulfite and the accelerator. Also the addition of a water adsorbent will improve the bonding char acteristics of the plaster. However, to be effective for the purposes of the invention the formulated special plaster should contain the soluble sulfite, and the accelerators and colloidal water adsorbents where added, within the fol lowing ranges:

Ingredient: Range, percent Soluble sulfite 0.005 to 2.0 Accelerator 0.1 to 10.0 Colloidal adsorbent 0.05 to 30.0

And best results have been obtained to date within the following:

Ingredient: Range, percent Soluble sulfite 0.05 to 0.40 Accelerator 0.2 to 3.0 Colloidal adsorbent 0.5 to 3.0

By way of a specific example, particularly good results can be obtained in bedding plate glass blanks on grinding and polishing tables with a plaster formulation made up of a cement based gypsum plaster including 95% um formulated plaster, 2.60% portland cement, 0.05% 50+ dium hydrogen sulfite, 1.85% potassium sulfate and 0.50% colloidal silica.

However, regardless of the specific formulation of the improved plaster composition of this invention, the essential ingredients are: (l) formulated or unformulated gypsum plaster and (2) a soluble sulfite additive.

Where a formulated commercially available gypsum plaster is used as the starting material, either a cement or a lime based plaster will be suitable and, where unformulated gypsum plaster is employed, portland cement of the air-entraining or other type is preferably added, usually in amounts between 1.75 and 3%.

To provide the soluble sulfite, any soluble derivative of sulfurous acid, the acid itself or a pyrosulfite may be employed, except that those that cause extreme retardation of the plaster set, due to the presence of specific ions causing such retardation with the sulfite or pyrosulfite ions contained in said substance, should generally be avoided.

The most practical soluble sulfites to use are the sulfite salts of the alkali metals and, of these, the hydrogen sulfite and the pyrosulfite are generally more efficient than the sulfite, especially in retarding the initial setting period.

This is graphically illustrated in FIGS. 2 to 4 of the drawings wherein the curves depict the action of varying amounts of sodium sulfite, sodium hydrogen sulfite and sodium pyrosulfite respectively in retarding the initial set of a commercially available plaster composition widely used in bedding glass blanks on grinding and polishing tables.

The manner in which these sulfites, with an accelerator, also act to shorten the total setting period by accelerating the final setting action is graphically depicted in FIG. 5 where the temperature rise for a typical commercial plaster, and for the same plaster with the addition of 0.125% sodium hydrogen sulfite and 2.0% potassium sulfate (based on the weight of the plaster used), is plotted against time in minutes.

The graphs of FIGS. 2 to 4 were obtained in the fol lowing manner:

(1) Varying amounts of each of the examples of the essential additive were added to successive samples of the plaster used.

(2) An equal mass of water at a standard temperature was added to the plaster.

(3) A standardized mixing procedure was used to simulate production practice.

(4) Gel point, initial and final set times (defined as follows) were obtained for each plaster sample.

The gel point was taken as the time elapsed from the mixing of the plaster and water until the Vicat plunger left a visible indentation in the slurry upon removal. (This is a significant factor because of the difliculty in working the plaster after this point has been reached.)

The initial set was taken as the time elapsed from the mixing of the plaster and water until the Vicat plunger failed to penetrate within 1 mm. of the bottom of the sample.

The final set time was taken as the time elapsed from the mixing of the plaster and water until the first of three successive, identical readings of the Vicat scale were obtained.

On this basis each of FIGS. 2 to 4 shows the retardation of initial set of a typical plaster containing varying amounts of one soluble sulfite. It will be noted in FIGS. 3 and 4 that a rather sharp maximum appears at low conccntrations of these sulfites. However considerable changes in both the degree of retardation and the range over which retardation occurs are possible when the pH of the plaster slurry is raised above 9 or when varying amounts of water adsorbent are present. In addition, sufficient retardation may be obtained in the portion of the curve beyond the maximum, thus allowing greater tolerance in the control of the additive content.

FIG. 5 demonstrates the over-all setting action of a typical plaster, with and without additives of the invention, by means of a temperature-time plot, a commonly used method of illustrating plaster setting characteristics. It will be seen, in this case, that the initial increase in temperature occurs first in the plaster without additives but that the rate of rise is considerably greater when an essential additive of the invention and an accelerator are included. In addition, final setting of the plaster with the additive and accelerator, as indicated by a leveling off in temperature, is completed more rapidly and at a higher temperature than in the case of the plaster without the additive and accelerator.

It should also be noted that, since an accelerator affects the concluding stages of the plaster setting process to a proportionately greater extent than it does the gel point, variation in the accelerator content of a given plaster, when coupled with varying amounts of the essential additive, permits considerable flexibility of control over the over-all plaster setting characteristics.

In most of the early work with the invention the sodium sulfites were used because of their low price and ready availability. However, the sulfite, hydrogen sulfite and pyrosulfite of the other alkali metals, such as potassium and lithium, will act in a very similar manner to the corresponding sodium sulfites as will most of the other soluble sulfites.

In the specific example and in FIG. 5 potassium sulfate is employed as the accelerator because this has given the best results. However, any inorganic chemical which will provide a neutral or acidic aqueous solution will function as an accelerator for gypsum plaster although the presence of the cement in the formulated plaster composition introduces variability into the result so that ammonium nitrate and hydrochloric acid, for examples, which are generally considered effective accelerators, are not suitable and may even act as mild retarders. After potassium sulfate, potassium nitrate and ammonium sulfate may be listed as the most eflective accelerators in the plaster composition of this invention.

Colloidal silica is employed as the water adsorbent in the specific example but any non-hygroscopic, hydrophyllic colloid that will function as an essentially neutral additive with respect to plaster set times can be used effectively.

The purpose of the water adsorbent material in the special plaster compositions is to improve the water retention of the soluble sulfite containing plasters. This is desirable because the soluble sulfites have a tendency to decrease the quantity of water retained in the set plaster and retained water is necessary for a good bond between the plaster and glass. Decreasing the water-plaster ratio is helpful from the standpoint of water retention but a point is soon reached where the early setting stages are undesirably accelerated. This is controlled in the preferred plaster compositions of the invention by the use of the colloidal water adsorbent.

In deter-mining the exact amounts of the additives to be used in formulating the special plaster, consideration must be given to such plaster variables as setting characteristics, age prior to use, the water to plaster mixing ratio and changes in the sources of supply of the basic plaster ingredients. However, the ranges of ingredients here set forth were arrived at with such variables in mind.

Generally speaking the use of lower concentrations of the soluble sulfites are favored both from a cost standpoint and because of the pronounced retardation effects which occur at such low concentrations. Thus as is graphically shown in FIGS. 3 and 4 of the drawings, maximum retardation normally occurs with between approximately .l% and 2% of the hydrogen sulfite and pyrosulfite and is about 6 minutes based on values for normal production slurries made with conventional plaster compositions. This figure will be modified by the use of the accelerator and/or the presence of a water adsorbent at elevated pH. The steepness noted in the graphs of the drawings in the retardation effect versus concentration curve shows that a very small change in the concentration of the soluble sulfite in dilute solutions changes the retardation effect and therefore the time of occurrence of the initial set to a considerable extent.

It is to be understood that the forms of the invention herewith described are to be taken as illustrative embodiments only of the same, and that various procedural changes, as well as various changes in composition, may be resorted to without departing from the spirit of the invention.

I claim:

1. In a method of finishing glass, the steps of laying said glass in a slurry comprising a liquid vehicle and a plaster composition consisting primarily of gypsum plaster to which has 'been added .005 to 2.0% by weight of a soluble sulfite selected from the group consisting of the alkali metal sulfites, hydrogen sulfites and pyrosulfites and then forcing said glass into said slurry prior to subjecting the glass to said finishing.

2. A method as defined in claim 1 in which from 0.1 to 10.0% by weight of a plaster setting accelerator selected from the group consisting of potassium sulfate, potassium nitrate and ammonium sulfate has also been added to the plaster composition.

3. A method as defined in claim 1 in which from 0.05 to 30.0% by weight of a colloidal adsorbent has also been added to said plaster composition.

4. A method as defined in claim 1 in which from 0.1 to 10% by weight of a plaster setting accelerator selected from the group consisting of potassium sulfate, potassium nitrate and ammonium sulfate and from 0.05 to 30.0% by weight of a colloidal adsorbent have also been added to said plaster composition.

5. A method as defined in claim 1 in which from 0.05 to 0.40% of the soluble sulfite has been added to the plaster composition.

6. A method as defined in claim 1 in which the soluble sulfite is a hydrogen sulfite.

7- A method as defined in claim 1 in which the soluble sulfite is a pyrosulfite.

8. A method as defined in claim 1 in which the liquid vehicle is water, the soluble sulfite is a sodium salt of sulfurous acid, from 0.1 to 10.0% by weight of a compound selected from the group consisting of potassium sulfate, potassium nitrate and ammonium sulfate is added as an accelerator to the plaster composition, and from 0.05 to 30.0% by weight of a hydrophyllic colloid that will function as an essentially neutral additive with respect to plaster set times is also added to said plaster composition.

9. In a method of finishing glass, the steps of laying said glass in a Water slurry of a plaster composition consisting primarily of gypsum plaster to which has been added from 0.005 to 2.0% by weight of a sodium salt of sulfurous acid, from 0.1 to 10.0% by weight of potassium sulfate and from 0.05 to 30.0% by weight of colloidal silica and then forcing said glass into said slurry prior to subjecting the glass to said finishing.

10. A method of improving the setting characteristics of a plaster composition consisting primarily of gypsum plaster which comprises adding .005 to 2.0% by weight of a soluble sulfite selected from the group consisting of the alkali metal sulfites, hydrogen sulfites and pyrosulfites to said composition.

11. A method as defined in claim 10 in which said gypsum plaster is a cement based gypsum plaster.

12. A method as defined in claim 10 in which the soluble sulfite is a sodium sulfite.

13. A method as defined in claim 10 in which from 0.1 to 10.0% by weight of a compound selected from the group consisting of potassium sulfate, potassium nit-rate and ammonium sulfate is also added to said composition as a plaster setting accelerator.

14. A method as defined in claim 10 in which from 0.05 to 30.0% by weight of a hydrophyllic colloid that will function as an essentially neutral additive with respect to plaster set times is also added to said composition.

15. A method as defined in claim 10 in which from 0.1 to 10.0% by weight of a plaster setting accelerator selected from the group consisting of potassium sulfate, potassium nitrate and ammonium sulfate and from 0.05 to 30.0% by weight of a hydrophyllic colloid that will function as an essentially netural additive with respect to plaster set times is also added to said composition.

16. An improved plaster composition comprising gypsum plaster and .005 to 2.0% by weight of a soluble sulfite selected from the group consisting of the alkali metal sulfites, hydrogen sulfites and pyrosulfites.

17. A plaster composition comprising gypsum plaster from 0.005 to 2.0% by weight of, an alkali metal salt of sulfurous acid and from a 1 to 10.0% by weight of potassium sulfate.

18. A plaster composition comprising gypsum plaster from 0.005 to 2.0% by weight of, an alkali metal salt of sulfurous acid and from 0.05 to 30.0% by weight of a colloidal water adsorbent.

19. A plaster composition comprising gypsum plaster, from 0.05 to 0.40% by weight of a sodium salt of sulfurous acid, from 0.2 to 3.0% by weight of potassium sulfate and from 0.5 to 3.0% by weight of colloidal silica.

References Cited UNITED STATES PATENTS 1,898,358 2/1933 Gelder 106-110 2,340,840 2/1944 Wiss 106-110 2,358,701 9/1944 Gardner 106ll0 3,100,715 8/1963 Leonard l06-3l5 TOBIAS E. LEVOW, Primary Examiner. S. E. MOTT, Assistant Examiner. 

1. IN A METHOD OF FINISHING GLASS, THE STEPS OF LAYING SAID GLASS IN A SLURRY COMPRISING A LIQUID VEHICLE AND A PLASTER COMPOSITION CONSISTING PRIMARILY OF GYPSUM PALSTER TO WHICH HAS BEEN ADDED .005 TO 2.0% BY WEIGHT OF A SOLUBLE SULFITE SELECTED FROM THE GROUP CONSISTING OF THE ALKALI METAL SULFITES, HYDROGEN SULFITES AND PYROSULFITES AND THEN FORCING SAID GLASS INTO SAID SLURRY PRIOR TO SUBJECTING THE GLASS TO SAID FINIGHING. 